Two catalysts of tungsten sulfide supported on an ultra-stable Y zeolite were prepared by impregnation with aqueous solutions of ammonium metatungstate at acidic (2.7) and basic (11) pH values. The catalysts were characterized by HRTEM, SEM-EDX and FT-IR of adsorbed NO to determine if the location and dispersion of the WS2 particles is changed after the activation of the catalyst, and what consequences this has on the catalyst activity during the HDS of gasoil. W-2.7 catalyst showed a higher concentration and homogeneous distribution of the WS2 structures on the external surface of the zeolite support, and a decreased amount within the zeolite cavities with respect to W-11. The catalyst prepared at pH=2.7 exhibited higher activity in the HDS of gasoil. The characterization results indicate that because of the size of the sulfur molecules present in the gasoil, their hydrodesulfurization occurs mainly on the external surface of the zeolite, where this catalyst displays a superior dispersion of the WS2 species.